How to make Git pull use rebase by default for all my repositories?

How about

git config --global pull.rebase true

This will tell git to always pull with rebase.

There are now 3 different levels of configuration for default pull behaviour. From most general to most fine grained they are:

1. pull.rebase

Setting this to true means that git pull is always equivalent to git pull --rebase (unless branch.<branchname>.rebase is explicitly set to false). This can also be set per repository or globally.

2. branch.autosetuprebase

Setting this to always means that whenever a tracking branch is created, a configuration entry like the one below will be created for it. For finer grained control, this can also be set to never, local or remote and can be set per repository or globally. See git config --help for further details.

3. branch.<branchname>.rebase

Setting this to true means that that particular branch will always pull from its upstream via rebasing, unless git pull --no-rebase is used explicitly.

Conclusion

So while you can't change the default behaviour for all future clones of a repository, you can change the default for all of the current user's (existing and future) repositories via git config --global pull.rebase true.